Human cytomegalovirus (HCMV) causes severe disease in the immunocompromised (AIDS and transplant patients, and neonates). It is also associated with chronic human diseases in the immunocompetent. A critical feature of HCMV infections is the hematogenous dissemination of the virus that is central to disease pathogenesis. HCMV viremia is cell-associated, but the cell type responsible has not been conclusively identified. Monocytes are likely candidates to mediate viral dissemination. Monocytes are the primary cell type infected in the blood and are the predominant infiltrating cell type found in infected organs. The problem is that monocytes are not productively infected by HCMV; virus replication is abortive. Macrophages (differentiate from monocytes) support productive viral replication and infected macrophages are found in multiple tissues in HCMV patients. However, they are not blood-borne cells and cannot be responsible for hematogenous spread. Thus, current findings are contradictory and the cell type involved in HCMV spread remains unresolved. We propose a solution to this apparent conundrum and that is, that HCMV infection of monocytes drives their specific migration out of the blood into peripheral tissues, where they can differentiate into macrophages and can then replicate the original virus carried from the blood into the tissue by the infected monocytes. Therefore, we suggest that HCMV possesses the ability to drive extravasation and monocyte-to-macrophage differentiation and that this is a critical function of the virus. We have preliminary data to support this proposal. To our knowledge, a precedent for this strategy in other viruses has not been reported and it is consistent with what is known about HCMV pathogenesis. We hypothesize that HCMV induces extravasation of monocytes to peripheral tissues and their differentiation into productively infected macrophages. The specific aims are as follows: 1) to investigate if viral binding initiates monocyte-to-macrophage differentiation and the role of HCMV infection in monocyte extravasation; 2) to examine viral gene expression and replication in viral differentiated macrophages; and, 3) to determine the molecular signaling mechanisms associated with the viral-induced differentiation of monocytes into macrophages. Overall, this proposal investigates the mechanisms involved in HCMV spread and persistence. [unreadable] [unreadable]